There are a couple of points which are/aren't related.
First is the issue of bundling software. As it's already been stated, everyone does it. Sure they could use a 3rd party option, but it doesn't make good business sense to do so. If Mozilla had a very sucessful OS, I bet every dollar I have that they would bundle Firefox with it.
As it's already been said, chrome and even Firefox have been quite successful, despite the various browsers being built in to their respective OSes. So the point that you cannot be successful in this regard is false. In order to be sucessful, you simply need to have a better product.
Secondly is the point of privacy and ethics, but again its well known that companies in general are always looking out for their bottom line. Despite all this... all the various OSes allow any user to install which ever browser they want to use. If you don't like MS, or Google, then install Firefox, etc... If these companies were preventing users from installing a browser of their choice, then that's a different story.